The demand for image input image sensors, such as CMOS (complementary metal-oxide-semiconductor) image sensors or CCD (charge coupled device) image sensors, in the application fields of digital cameras and cellular phones equipped with cameras, has increased along with the performance improvement of such image sensors. Image sensors with improved performance characteristics are desired. One such characteristic is expansion of the dynamic range. For such image sensors, further improvement in characteristics is demanded, and one of the characteristics is the dynamic range that should be expanded.
Examples of solid-state image pickup devices designed for realizing a wider dynamic range are given in Japanese Kokai Patent Application No. 2003-134396, Japanese Kokai Patent Application No. 2000-165754, Japanese Kokai Patent Application No. 2002-77737, and Japanese Kokai Patent Application No. Hei 5[1993]-90556. However, for such solid-state image pickup devices, a wide dynamic range is difficult to realize while maintaining high sensitivity and a high S/N ratio.
To address this problem, another type of solid-state image pickup device, shown in Japanese Kokai Patent Application No. 2005-328493, was developed. In that solid-state image pickup device, the structure is such that photoelectric charge overflowing from the photodiode of each pixel is stored in a floating diffusion and an electrostatic capacitive element. When photoelectric charge does not overflow from the photodiode, the photoelectric charge in the photodiode is used to obtain the signal of each pixel. On the other hand, when it overflows, the sum of the photoelectric charge in the photodiode and the photoelectric charge that has overflowed from the photodiode gives the signal of the pixel. When such solid-state image pickup device is manufactured using the CMOS process, however, the dark current component with respect to the photoelectric charge that has overflowed from such photodiode increases by about 3-4 orders of magnitude higher than the required level. Consequently, it is inappropriate for use for storage of photoelectric charge over a long period of time, and there is a demand for development of a scheme to suppress such dark current. For example, the sites of generation of the dark current component include the interface directly below the gate of the transistor, the side surface of the element separating insulating film, the portion of the silicon surface contacting the depletion layer, etc.
International Patent Application Publication No. WO 2005/083790, Japanese Kokai Patent Application No. 2005-328493, and Japanese Kokai Patent Application No. 2006-217410 disclose solid-state image pickup devices that can suppress the dark current component and can expand the dynamic range while maintaining high sensitivity and a high S/N ratio. And, for these solid-state image pickup devices, efforts have been made to realize high sensitivity and a high S/N ratio especially in the low-luminance region.
The problem to be solved is how to realize high sensitivity and a high S/N ratio, especially in the low irradiation region, while maintaining a wide dynamic range.